Flame retardant polymer compositions are well-known in the art. Because of potential environmental and toxicity concerns, there is increasing interest in antimony and halogen-free systems which provide the desired flame retardant properties to a polymeric article. Phosphorous-based compositions are shown, for example, in Publication Nos. WO 99/43747 and WO 97/41173.
The '747 publication entitled Flame Retardant Polymer Blends discloses an antimony-free combination of a phosphate-based flame retardant, especially an aryl diphosphate, and an organoclay which are reported to afford synergistic flame retardant properties to polymer blends, especially polyphenylene ether-polystyrene blends and blends of polycarbonates with styrene copolymers such as ABS copolymers. The polyphenylene ether-polystyrene blends free from fluorocarbon additives are preferred according to the disclosure.
The '173 publication entitled Flame Retardant Composition for Polymers claims a flame retardant composition, adapted to be mixed with a polymer substrate to confer flame retardancy on the substrate, which comprises: (a) a bicyclic phosphorus flame retardant compound, such as one containing one or more pentaerythritol phosphate alcohol moieties, as exemplified by bis(pentaerythritol) phosphate alcohol carbonate; (b) an intumescent flame retardant compound containing nitrogen and phosphorus, such as melamine phosphate; and (c) a monophosphate ester compound to enhance the charring and processing characteristics of the composition in the polymer substrate, such as a liquid aryl group containing phosphate ester compound. The monophosphate ester compound is typically triphenyl phosphate.
See, also, U.S. Pat. No. 4,454,064 to Halpern et al. which discloses a method for the preparation of pentaerythritol phosphate by mixing pentaerythritol and phosphorous oxychloride in a solvent and heating to a temperature of 75 to 125° C., as well as U.S. Pat. No. 5,237,085 to Telschaw et al. which teaches synthesis of pentaerythritol-based phosphorous heterocycles by reaction of a pentaerythritol polyol with either a trivalent or pentavalent compound using an aryl phosphate solvent at an elevated temperature. A variety of bicyclic phosphate compounds are likewise disclosed in U.S. Pat. No. 3,293,327 to Hechenbleikner et al.
Intumescent flame retardant compositions including 2,6,7-trioxa-1-phosphobicyclo[2.2.2.]octane-4-methanol-1-oxide and a nitrogen compound selected from the group melamine, ammeline, benzoguanidine, guanidine, urea and salts thereof, are reported to be intumescent and readily adapted to flame retard a variety of dissimilar resins including polyolefins, polyvinylaromatic resins, polycarbonates, polyacrylates, polyamides, PVC and blends thereof in U.S. Pat. No. 4,341,694 to Halpern.
U.S. Pat. No. 5,204,393 to Nalepa et al. describes a flame retardant intumescent polyolefin which comprises a combination of ammonium polyphosphate, tris(2-hydroxyethyl) isocyanurate; melamine cyanurate; and a selected silica in an amount from 0.5 percent to an amount equal to one-half the amount by weight of tris(2-hydroxyethyl) isocyanurate.
Intumescent polymer systems generally are believed to function by way of (1) a carbonific or char-forming component which can be the polymer itself, (2) an acid generating component which acts as a catalyst and (3) a spumific or blowing agent, typically a nitrogen source of both ammonia and N2. The greater the char volume, the better the insulating, whereas the inner cross-sectional core should be cellular and close celled. In addition, the surface crust should be thick, continuous and impenetrable. See Scharf, D. J., Intumescent Fire Retardants for Plastics—A Continuance (undated). The catalyst or acid generating component is frequently a phosphorous derivative, the function of which is to catalyze dehydration. See Lewin, M., Some Aspects of Synergism and Catalysis in FR of Polymeric Materials—An Overview, Ninth Annual BCC Conference on Flame Retardancy, Business Communications Co., Inc. Norwalk Conn., wherein ammonium polyphosphate is described as the acid generating component and nitrogen source in a polypropylene system where the polymer itself is the carbonific or char-forming agent.
Perhaps more typically, an organic polyol is used as a carbonific and melamine or a melamine derivative as a spumific. See Scharf, D. et al, Studies on Flame Retardant Intumescent Char: Part 1, Fire Safety Journal 19, pp. 103-117, Elsevier (Ireland, 1992). In this publication, it is reported that titanium dioxide in suitable amounts exerts a reinforcing or synergistic flame retardant effect, whereas stannous oxide is antagonistic. Likewise, it has been reported that zinc and manganese salts can benefit APP performance. See Lewin et al., Mn and Zn Compounds as Catalysts of Intumescent Flame Retardancy of Polypropylene, May, 2000, BCC Conference.
In any polymer system, it is desirable to limit or minimize expensive additives from a cost perspective. In addition, additives can have adverse effects on processing or properties which, of course, becomes more pronounced as the additive load in the system increases. The commercial success of intumescent polymer systems has clearly been limited by the high loading of acid generating component and spumific required to achieve the desired (typically 94V-O) rating in terms of flame resistance.